A visual illusion (also called an optical illusion) is characterized by visually perceived images that differ from objective reality. The information gathered by the eye is processed in the brain to give a perception that does not tally with a physical measurement of the stimulus source. There are three main types: literal optical illusions that create images that are different from the objects that make them; physiological optical illusions that are the effects on the eyes and brain of excessive stimulation of a specific type (brightness, color, size, position, tilt, movement); and cognitive optical illusions, the result of unconscious inferences.
Motion perception is responsible for a number of sensory illusions. Film animation is based on the illusion that the brain perceives a series of slightly varied images produced in rapid succession as a moving picture. Likewise, when we are moving, as we would be while riding in a vehicle, stable surrounding objects may appear to move. We may also perceive a large object, like an airplane, to move more slowly than smaller objects, like a car, although the larger object is actually moving faster. The Phi phenomenon is yet another example of how the brain perceives motion, which is most often created by blinking lights in close succession. The perception of motion can also be created by what has been termed “reversed phi,” in which two lights of opposite contrast polarity are alternated to create the appearance of motion in the direction opposite to that predicted by Phi. Others have used phi and reverse phi to create the appearance of continual motion (i.e., illusory motion perpetually moves in one direction) by juxtaposing two images and slightly offset negatives (referred to as four-stroke motion) or by inserting a gray frame between two slightly offset lights (referred to as two-stroke motion). In addition, non-continual motion can be created by modulating the luminance of thin lines at edges surrounding objects: if a gray rectangle is bordered on the left by a thin white line and on the right by a thin black line, then modulating the luminance of a field surrounding the rectangle will make the rectangle appear to shift back and forth; i.e., when the surrounding field is bright, the rectangle appears to shift to the right, and when the surrounding field is dark, the rectangle appears to shift to the left. The motion arises even though the rectangle and the edges are physically stationary.
Here we present two types of visual displays that lead to the perception of perpetual motion but do not create changes in physical space—that is, continual motion from physically stationary objects. In other words, the displays combine the perceptual motion found in reverse phi phenomena with the thin edges found in edge motion conditions. The key insight into these conditions is that motion signals can be created by modulating the luminance of thin edges in relation to the phase of luminance modulation of fields that surround the edge. When viewing displays that combine opposite direction motion signals, the visual system will group the display into the perception of a moving object. A motion signal is created by changing the modulation at the edge of the field.
Traditional vision acuity tests have used static optotypes as displays of printed or projected characters, objects, or shapes. Numerous patterns, configurations, and methods for static optotypes have been proposed for testing acuity based upon the ability of a subject to distinguish these various shapes, sizes, contrasts, and colors in tests such as Snellen charts, tumbling “E” arrays (static images of the letter “E” where the static image is also rotated 90 degrees, 180 degrees, and 270 degrees for discernment), Landolt “C” charts, and so on. Certain prior art vision testing patterns use periodic images, such as disks, rectangles, diamonds, etc.; others are quasi-periodic, such as tri-bar, and small checkerboard designs.
While the Landolt “C” chart is the clinical standard for acuity, the familiar Snellen eye testing chart as developed in 1862 using large, black, serifed letters on a white background is the test frequently used for determining visual acuity. The concept of these charts to verify acuity is based upon the patient seeing patterns such as letters or printed images on those charts. Snellen's standard is that a person should be able to see and identify a 3.5 inch letter at a 20 foot distance (that ratio being consistent regardless of its use in the “English” or Metric system). A disadvantage of the Snellen-type images is that even defocused letters can still be partially recognized by their blur patterns. Much time is thus wasted as the patient, whose eyes are being tested, attempts to guess the letter. The design of the Snellen chart is further complicated by each letter having a different degree of recognizability and by the tendency of the patient to strain to perceive coherency when trying to identify the letters.
Thus, most visual testing systems are intended for optometry offices where there are precise optical devices that can be used to measure visual function with conscious articulate observers. However, it is often important to give quick assessment of visual function in active environments, such as for sports activities, military training or in conditions of high attention load, or for observers who are unable to respond in conventional ways (e.g. infants, non-communicative severely disabled patients or patients suffering from head trauma or other forms of dementia, or observers who are intentionally trying to deceive the tester (malingerers), or sometimes there may be the desire to test observer acuity covertly or without observers being aware that they are undergoing an examination.
The visual displays presented here have three features that make them useful for optical testing in situations other than in the standard optometry setting with conscious articulate observers: 1. The illusory motion depends upon the appearance of edges that change their luminance levels over time (i.e. they change from light to dark and back to light). For observers with normal visual acuity, the edges can produce the appearance of motion even when the edges are remarkably thin. Observers with normal visual acuity can see motion when the edges are as thin as 0.1 min of visual angle, but the poorer an observers visual acuity the thicker edges need to be in order to see the illusory motion. As a general rule, if observers can discern the edges, then they will be able to see motion. Hence, the appearance of motion can be taken as a measure of an observer's ability to see detail. 2. The perpetual motion created by the illusion drives eye movements in the direction of the motion even though all objects in the display are physically stationary. Observers' eye movements therefore give an indication of whether or not the observers are able to discern the edges. 3. The illusions (and eye-tracking) can be displayed on any monitor system and in conjunction with other images. The illusions (and eye-tracking) can therefore be displayed on phones, computer monitors, virtual reality headsets, etc., to test visual function.
These three features of the visual displays presented here allow visual function to be assessed in a wide variety of conditions where assessment is desired, but verbal (or gestural) response is impossible or misleading. This may occur, for instance, with elderly, dementia, or infant populations, or conditions with high attentional load (pilots or sports), or conditions in which a verbal response may be deceptive (for instance, people trying to be excused from military service by claiming poor eye sight). Our new eye-tracking test eliminates issues from non-verbal, non-gestural, and deceptive observers, as eye movements in the direction of the illusory motion will occur naturally and involuntarily when the movement is seen.